Method of vapor sealing anodically produced oxide films on aluminum and aluminum alloys



United States Patent US. 'Cl. 1l7106 6 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to scaling a porous anodized coating on a body ofaluminum or aluminum alloy by exposing the coating to mixed vapors ofwater and a chlorinated hydrocarbon such as trichloroethylene orperchloroethylene.

This invention relates to the anodizing of aluminum and aluminum alloys,and particularly to a method of sealing a porous anodized coating onaluminum, the latter term as employed hereinafter including alloys inwhich aluminum is the predominant constituent.

Oxide coatings produced on aluminum by anodizing are usually porous andhave to be sealed if highest corrosion resistance and other desirableproperties are to be achieved. Exposure of the anodized surface to steamin closed chambers is generally accepted to produce the best sealingeffect, but steam scaling is a time consuming operation. The sealingtime can be reduced to some extent by the use of superheated steam, butthe high pressure of the superheated steam must be contained in costlyvessels so that this method has not found wide acceptance.

It is a common drawback of all steam sealing methods that they operateat or above the boiling point of water so that the steam in the sealingchamber must be vented to the atmosphere or condensed before a batch ofscaled material can be removed from the chamber to avoid scalding theoperator. The resulting consumption of steam and of thermal energy makesthe process economically inefficient.

It has now been found that vapors of chlorinated hydrocarbonssubstantially saturated with water produce an adequate or even asuperior sealing effect even at temperatures lower than 100, and thatsuch a sealing process can be performed very adequately in a containeropen to the atmosphere. The process of the invention produces adequatesealing in a fraction of the time required for steam sealing at about100 C., and the thermal efficiency of the new process is high.

The mixture of water and chlorinated hydrocarbon vapors can be preparedby jointly heating water and the liquid hydrocarbon to the necessarytemperature. The two liquids being practically insoluble in each other,it is preferred vigorously to stir or otherwise to agitate the liquidswhile they are being heated so that water and the hydrocarbon arepresent in the gas phase in vapor pressure equilbrium with the liquidphases.

A gaseous mixture of water and chlorinated hydrocarbon in which anodizedaluminum may be sealed quickly and completely is also produced byintroducing saturated, and preferably superheated steam, into a body ofliquid chlorinated hydrocarbon. The steam partly evaporates the solventand provides the mechanical agitation necessary for equilibriumsaturation of the gas phase with the constituent elements.

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It is also possible to evaporate the chlorinated hydrocarbon by suitablysupplied thermal energy, to feed steam into the solvent vapor near thesurface of the hot liquid, and to stir the liquid while an anodizedaluminum object is immersed in the layer of relatively heavywater-bearing solvent vapor.

The chlorinated solvents containing more than one chlorine atom permolecule and having a boiling point near that of water, that is, atl00i25 C. have been found to be most effective. Trichloroethylene andperchloroethylene, the conventional dry cleaners solvents, meet theserequirements and are readily available at low cost. The rapidly producethe desired results in the sealing method of the invention.

The effect of sealing on freshly anodized porous aluminum surfaces isreadily evaluated by a dye absorption test. Aluminum anodized by mostconventional methods, when not sealed, avidly adsorbs most wool dyes,particularly acid dyes, but also certain dyes which do not readily colorwool. Fully sealed aluminum does not take up such dyes from an aqueoussolution, and the ability of anodized aluminum to adsorb or absorb acertain green dye is the basis of a widely accepted standardized sealingtest (see Aluminum, 38 (1962), 150-154 and 161). It permits anapproximate quantitative evaluation of the efiiciency of a sealingmethod, and was employed in the following examples which furtherillustrate this invention.

EXAMPLE 1 An upright cylindrical container having a capacity of about200 liters and an open top was charged with 25 liters distilled waterand 25 liters perchloroethylene. A fairly uniform mixture of the twoliquids was maintained in the container by vigorously stirring with anagitator driven at high speed by an electromotor.

A steam coil was immersed in the liquid and controlled by a thermostatarranged in the vapor phase above and set for -90 C. A cooling coilupwardly spaced from the liquid surface vertically confined the vapor bycondensation and made it unnecessary to cover the open top of thecontainer.

The vapor of perchloroethylene saturated with water quickly reached astate of equilibrium. Cups of 300 ml. capacity made from commercialaluminum metal (99.5% Al) were anodically coated with a thin oxide layerin a conventional sulfuric acid electrolyte, carefully rinsed, andimmersed in the vapor layer between the liquid surface and thecondensation zone on the level of the cooling coil. Groups of sampleswere withdrawn every minute and tested for dye adsorption.

Excellent sealing was achieved within eight minutes. percent of thetested samples gave a negative dye adsorption test after only sixminutes.

Steam sealing tests performed at ambient pressure with anodized cupsfrom the sample batch indicated that a sealing time of more than 20minutes was necessary to achieve sealing comparable to the resultsachieved in 8 minutes in the gaseous mixture of water andperchloroethylene.

EXAMPLE 2 The procedure of Example 1 Was repeated with trichloroethylene instead of perchloroethylene, and operating at atemperature of 70-80 C. The results obtained were similar to thoseachieved with perchloroethylene, but the sealing period required wassomewhat longer than was necessary with perchloroethylene, twelveminutes being required on an average for complete sealing.

Analogous results were obtained with all commercial aluminum alloyswhich are usually anodized, and the alloying elements effected sealingwith moist chlorinated hydrocarbon vapors in the same manner in whichthey are known to affect steam sealing or sealing in boiling water withor without addition agents. The superiority of the solvent vapor sealingmethod over the known methods varied in magnitude from alloy to alloy,but was always significant.

The electrolyte employed in anodizing was not found to have asignificant effect on the relationship between the time required forsealing anodized coating by the method of the invention and the timerequired for steam sealing.

What is claimed is:

1. A method of sealing a porous anodized coating on a body of aluminumor aluminum alloy which comprises exposing said coating to mixed vaporsof water and of a chlorinated hydrocarbon, said hydrocarbon beingnormally liquid.

2. A method as set forth in claim 1, wherein the boiling point of saidhydrocarbon is between 75 C. and 125 C., each molecule of saidhydrocarbon containing at least two chlorine atoms.

3. A method as set forth in claim 2, wherein the temperature of saidmixed vapors is lower than the boiling point of said water and theboiling point of said chlorinated hydrocarbon, but not lower by morethan 25 C. than the lower one of said boiling points.

4. A method as set forth in claim 3, wherein said mixed vapor is incontact with liquid water and liquid hydrocarbon, and said liquid waterand hydrocarbon are agitated 4 at a rate sufiicient to substantiallymaintain vapor pressure equilibrium between said vapor, said water, andsaid liquid hydrocarbon.

5. A method as set forth in claim 3, wherein said vapor is produced byintroducing steam into a liquid body of said chlorinated hydrocarbon.

6. A method as set forth in claim 3, wherein said chlorinatedhydrocarbon is trichloroethylene or perchloroethylene.

References Cited UNITED STATES PATENTS 2,662,034 12/1953 Mason et al.117-49 2,837,451 6/1958 Hannon 1486.3 X 3,000,346 9/1961 Hnilicka117--106 X 3,365,377 l/1968 Michelson 204-35 OTHER REFERENCES MetalFinishing, vol. 54, No. 8, August 1956, pp. 53-57 relied upon.

ALFRED L. LEAVI'IT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

US. Cl. X.R.

